This invention relates to a process for the coating of solid particles.
It is known to coat solid particles or pellets in a rotating vessel especially using a tumbling or churning action to achieve uniform speed of coating material over the surface of the solid. We have found that greater uniformity of coating of solid particulate matter is obtained if during the coating process the solid particles are not tumbled or churned but roll or slide uniformly as a layer across a surface of rotation of a body rotating at high speed.
According to the present invention we provide a process for coating the particles of a particulate solid material, a process wherein
(a) the particulate solid material is supplied to a supply zone on the surface of a rotating body, the surface being a surface of rotation facing towards the axis of rotation of the body PA1 (b) due to the acceleration imparted by the rotation of the body the particles of the solid material are caused to travel across the surface of rotation from the supply zone to a discharge zone axially remote from the supply zone PA1 (c) coating material is brought into contact with the particulate solid and distributed over the surface thereof during its travel across the surface of rotation and PA1 (d) particles of the solid coated with coating material are discharged from the surface at the discharge zone using forces generated by the rotation of the body.
Examples of suitable rotating bodies are tubular vessels, conical or frusto-conical vessels preferably having a small deviation only from a cylindrical form, bowls or jars preferably having a curved concave surface as hereinafter described, their internal surfaces of rotation in all cases having a central axis. In such bodies the supply zone of the surface (which may be for example at or near to the central axis) and the discharge zone (which may be at or near to the circumference) will be axially displaced with respect to each other. The rotation of the body will generate forces which will operate on a particulate solid supplied to the surface, forces which include mainly a centrifugal force operating radially from the axis of rotation and a rotary force operating circumferentially at the surface of rotation. The rotary force overcomes any inertia of the solid particles on the surface and imparts a rotary motion to them due to frictional drag between the various surfaces. The surface of the rotating body is preferably a smooth, hard non-adherent surface; often a polished surface finished to a highly reflective surface is especially useful.
Without prejudice to the process of the present invention we believe that the centrifugal force operates in a direction away from the axis of rotation but perpendicular to that axis, and thus will tend to force the particulate solid towards the internal surface of rotation and thereby spread the particles of the solid across the surface. If a fresh supply of solid particles is provided to the supply zone of the surface whilst the body is rotating the spreading action of the centrifugal force will tend to operate away from that supply zone. If the particulate solid is supplied to the bottom of the rotating cup or jar the forces generated by the rotation, especially the centrifugal force, will form a layer of particles which will spread outwardly from the axis of rotation and move as a fairly uniform layer towards the rim of the cup or jar from which the particles will be discharged. This movement towards the rim of the cup or jar is a movement which is at least partially in an axial direction, i.e. upwardly if the jar is upright with a vertical axis and the particulate solid supplied to the base. However, the jar may be inverted and then the motion towards the rim will be downwardly.
The axis of rotation may be other than vertical although the vertical is the most convenient direction for the axis of rotation. The rotary propulsion may be applied to the body by a shaft from either end of the axis (e.g. from the top or the bottom if it is a vertical axis) or by other means e.g. turbo-propulsion.
The magnitude of forces generated by the rotation of the body which operate upon the particulate solid to move it across the surface will be dependent upon many factors which include the speed of rotation and the radius of the surface of rotation. The greater the speed of rotation and the greater the radius the larger is the centrifugal force.
The rate of supply of particulate solid to the surface of rotation is also a factor which affects the process. The faster the supply the thicker is the layer formed on the surface assuming other factors such as the particle size and speed of rotation are not varied.
The product may be discharged from the surface in a variety of ways for example by a scoop or knife blade in contact with the surface at the discharge zone or by sucking particles from a well. The preferred method is to use the centrifugal force to throw the products from the surface. It is convenient to place collecting apparatus around the rotating body to receive the product, the apparatus taking whatever form is appropriate to the type of product obtained. Electrostatic charges may be used as an aid to the collection of products, especially of fine particles.
The particulate solid may consist of small or large particles for example ranging from powders with particle dimensions of ca. 10.mu.-1000.mu. diameter up to prills and pellets of several millimeters in diameter. The solid is preferably a free-flowing solid not apt to cake or block i.e. the particles have little tendency to adhere one to another or to surfaces with which they come into contact. Thus the particulate solid (which may ideally be granules or prills of a synthetic product for example a drug, medicament, fertiliser, pesticide or foodstuff, or other chemical active in the biological sphere) will be normally in a dry condition and will slide or roll uniformly over the surface of the rotating body as a solid layer of moving particles in the process of this invention. However, cohesive powders and granules can be advantageously handled by this process and the forces generated by the rotation are used effectively to break down the cohesiveness and spread the particles uniformly on the surface.
Many different shapes of particles may be successfully moved across the surface as the body is rotating; some faster than others. For example spherical particles may be especially rapid and require very little acceleration imparted to them. However, the fastest particles may not be the easiest to coat uniformly: it is desirable to be able to obtain a mono-layer of close packed particles over at least part of the surface of rotation and in some cases this is easier to obtain when particles are slow moving.
The shape of the surface of rotation on which the solid particles are moved may be designed to make it relatively easy to obtain a mono-layer of close packed particles. An internal surface of a vessel of circular cross-section and having concavity in the axial direction so that the sides become steeper as the particulate solid moves nearer to the discharge zone is a preferred shape. Thus the particles are moved radially outward from the supply zone quickly on the shallow portion of such a surface but their progress is slowed down on the steeper portions of the surface. The layer becomes more uniform as the radial movement slows down but further supplies of particles from the supply zone continually arrive to maintain a steady flow as the uniform layer across the surface towards the discharge. The preferred shape for the surface often approximates to a parabola in the axial direction. The angle which the surface makes with the axis becomes smaller and tends to zero towards the discharge zone but in this preferred form the angle never equates to zero. We prefer a surface in which the curvature in the axial direction is such that the rate of increase in the diameter of the surface of rotation slows down uniformly towards the discharge zone.
The coating material may be a fluent-solid, a molten solid, a liquid or a fluid emulsion but it is preferably a solution of a solid in a liquid solvent. The coating laid down may be a film-forming polymeric material which would form an encapsulating layer or skin over each of the particles of the particulate solid to be coated or it may be a powdery solid which on evaporation of the solvent would form a powdery or sintered coating on the particulate solid. In this latter form the two solids should be adherent so that the coating remains on the particulate solid and to this end an adhesive material e.g. a viscous liquid or gum may be added to the coating solution. In certain circumstances the coating is an active material and for reasons of handling or dispersion it may be often useful to lay it over a non-active core or substrate.
The coating material may be supplied in a variety of ways the more convenient being a spray or a jet directed to the surface on which the particulate solid is moving. The spray may be an atomised spray produced for example from a rotating cup, disc or cove or by an electrostatic spray device.
Another preferred method is to apply the coating as a thin layer of coating material which is brought into contact with the solid particles to be coated, preferably as they are about to be discharged from the surface of rotation on which they are moving. The thin layer of coating material is conveniently produced on a second rotating surface, the periphery of which approaches close to or touches the rim of the vessel on which the solid particles are spread. We prefer the second rotating surface to be the surface of a stiff yet resilient member for example a member in the form of a core, disc or saucer suitably mounted co-axially with the other surface of rotation and arranged to be rotated independently thereof. For many applications it is preferable to rotate the two in opposite directions or at very different speeds. In a preferred embodiment the coating material in the form of a solution is applied to the surface of a rotating cone of flexible material whose modulus and dimensions are chosen to give the requisite stiffness, for example an 80 mm diameter cone of oriented polyester film of thickness ca. 0.4 mm is quite suitable or a thinner stainless steel sheet of ca. 0.1 mm may be used. The circumference of this cone is arranged to press lightly against the rim of the bowl from which the solid particles are about to be discharged. Thus the coating solution on the flexible cone is transferred to the solid particles by a rolling action of one surface over the other in the discharge zone, the surfaces being resiliently urged together in order to prevent solid particles being discharged without being coated.
A low partial pressure for the volatilisation of solvent is sometimes desirable and this is readily achieved in our process by enclosing the rotating body in an evacuated chamber for example a chamber enclosing an atmosphere at a pressure in the range from 10 mm mercury down to 0.01 mm mercury. Heat and/or air circulation or an inert atmosphere may also be provided if necessary.
A multi-stage process may be operated by arranging for example a series of rotating bodies, preferably rotating about the same axis and also conveniently from the same driving unit. The product from the first body is discharged to the surface of the second where the second process is conducted and so on to the third or other successive bodies. One may arrange for different coating materials for each stage and build up multi-layer coatings on the original particulate solid.
The process of the invention employs rotation at high speeds and by "high speed" as used herein we mean speeds in excess of 100 rpm. The forces operating on the particulate solid causing it to move on the surface do not depend solely on the speed but they depend also on the physical dimensions of the rotating body e.g. the radius and angle the surface makes with the axis of rotation. Thus any choice of speed of rotation must be selected bearing in mind the size and shape of the surface of the body which is rotating. The centrifugal force may be measured by the acceleration of the particles on the surface of rotation. Our process works most effectively when accelerations of at least 50 m sec.sup.-2 preferably at least 400 m sec.sup.-2 are present on the surface of rotation.
The speed of rotation may vary over a wide range, in general speeds from 100 rpm up to 100,000 rpm are used but the range which is most useful is from 500 rpm to 10,000 rpm. The design of rotor bearings and rotating seals and feeds for the rotating body may be those known conventionally in engineering science.
The surface of rotation on which the coating process is conducted is preferably an "unbroken" surface by which we mean a surface which is circumferentially continuous without holes, gaps or ports or other discontinuities, but the surface may be ridged, undulating, convoluted or otherwise patterned if desired in order to improve the movement of the particulate solid and/or the coated product.
The surface of rotation on which the coating process of this invention takes place may be usefully an inert surface for example of glass, plastic, ceramic, metal or composite materials especially chemically resistant metals e.g. stainless steel, nickel, titanium and tantalum are preferred.
Flexible films may be used as materials for the rotating bodies which would be rigidified by the centrifugal force operating when they are rotating and they may constitute a cheap form of bowl or cylinder on which the process may be performed.